Enhanced recovery of gold

ABSTRACT

An improved method for recovering refractory gold from a material comprising treating the material to at least partially remove nitric acid-insoluble lead moieties.

FIELD

This invention relates to methods of enhancing the recovery of gold froma material. In particular the invention relates to the recovery ofrefractory gold not normally amenable to cyanidation. Indeed as thepresence of refractory gold is not always evident the invention providesa method for the identification and recovery of refractory gold.

BACKGROUND

Cyanidation is frequently used to recover gold from source materials;however in some materials a proportion of the gold or even a majority ofthe gold content may not be amenable to direct cyanidation. The abilityto properly identify and recover refractory gold can represent asignificant economic benefit. A “refractory” gold containing material isa gold containing material such as an ore or a processed ore fractionwherein at least some of the gold cannot be liberated by standardcyanide processing. These refractory materials often requirepre-treatment in order for cyanidation or other treatments to beeffective in recovery of the gold. Examples of known refractory oresinclude sulphide containing minerals, carbon containing ores, ormaterials containing both sulphide minerals and carbon. Sulfide mineralsoften trap or occlude gold particles, making it difficult for the leachsolution to complex with the gold. Carbon present in gold ore may adsorbdissolved gold-cyanide complexes in much the same way as activatedcarbon, thereby reducing the amount of gold that can be recovered fromleach liquor. Gold that is present but is not amenable to directcyanidation is referred to as refractory gold.

Refractory gold materials may include raw ores, concentrates, flotationconcentrates, mine and process tailings and cyanide tailings andresidues. These materials may be refractory because precious metals arebonded to minerals in the ore, for example in a silver-manganese dioxideore, silver may be bonded to manganese and direct cyanidation does notsubstantially break this bond. Source materials may also be refractorybecause precious metals are bound or encapsulated in other minerals. Oneexample of this is the instance where precious metals are finelydispersed in sulphide ore so that there is little surface area ofprecious metal exposed to cyanide leach liquor. The recovery of preciousmetals from such ores may involve hydrometallurgical orpyro-metallurgical techniques or fine grinding prior to the cyanidationstep. Such pre-treatments are designed to render refractory goldamenable to dissolution in a subsequent (frequently cyanide) leach step.

Hydrometallurgical techniques of improving gold recovery includeleaching with sulphuric acid, hydrochloric acid or nitric acid.Treatment of source material with bacteria that oxidise sulfidic ores isalso possible. Pyrometallurgical techniques for improving gold recoveryinclude roasting, pressure oxidation and chlorination. The greatmajority of such techniques involve matrix erosion (and frequentlymatrix oxidation) of the source material. In other words, a largeproportion of the gold containing material is eroded in the effort torecover further gold. Matrix erosion, matrix oxidation and/or chemicaltransformation of the matrix material adds significantly to the cost andwaste associated with the process and may render such processeseconomically or environmentally unacceptable. Typically over 20% byweight of the matrix material in a process stream is eroded ortransformed by treatments to liberate refractory gold from that stream.

Gold in an ore may be present in parts per million, so that samplingerrors can be multiplied dramatically. It is well known that even smallinconsistencies in source materials (e.g. regions of high localconcentration of gold) can lead to dramatic errors when multiple samplesare taken and analysed for gold (this is sometimes called the nuggeteffect). A well-developed and practical method for identifying thepresence of refractory gold is the method of diagnostic leaching.Diagnostic leaching has been discussed by Lorenzen and Deventer inMinerals Engineering, Vol 6, Nos 8-10, 1013-1023 (1993), “TheIdentification of Refractoriness in Gold Ores by the SelectiveDestruction of Minerals”, the contents of which are incorporated byreference. In the diagnostic leach method, specific components in asample are first eliminated using a selective (e.g. oxidative) leach,and cyanidation is then used to extract the precious metal liberated bydestruction of this component. The precious metal extracted can bemeasured in solution to give a fairly accurate record of the amount ofthe precious metal associated with that component. Furthermore, theentire residue from this first leaching stage can be subjected to one ormore further selective leach stages, leading to the destruction offurther components that may prevent the extraction of gold. Washes withat least one of dilute acid and cyanide between these stages can also beused to destroy surface deposits, and to quantify the amount ofrefractory gold held up in the components that have been destroyed byselective leaching. The procedure can be varied to suit the mineralogyof the matrix material (i.e. the gold containing material). It iscustomary to quote assay results obtained from leach liquors as afraction (e.g. percent, or parts per million, or other fraction) ofwater-washed and dried starting sample. In this way all assay resultsare referred to the starting sample and can be directly compared.

Diagnostic leaching is not limited to ores and residues, but can dealwith any type of intermediary product that occurs on a plant.

Lorenzen and Deventer note that one leach in a diagnostic leach sequencecan be a nitric acid leach, followed by a cyanidation step. The nitricacid is said to decompose all ore matrix materials except silicates, andsome gold may remain locked in these silicates. This silicate residue isdescribed as “the ultimate residue”, because of the belief that allfurther gold in the sample is held up inside a silicate matrix. Atypical diagnostic leach procedure is:

-   -   (a) cyanide washing to liberate precipitated gold;    -   (b) cyanide leaching to liberate free gold;    -   (c) HCl/cyanidation to liberate gold associated with pyrrhotite,        calcite, ferrites etc;    -   (d) H2SO4/cyanidation to liberate gold associated with labile        copper sulfides, labile pyrite, base metal sulfides, uraninite        etc;    -   (e) HNO3/cyanidation to liberate gold associated with pyrite,        arsenopyrite, marcasite;    -   (f) inter-stage acid washes to liberate surface coatings; and    -   (g) acetonitrile elution to liberate gold absorbed on carbon,        kerogen, coal.

Henley et al. in Minerals Engineering Vol 14, Issue 1, January 2001,pages 1-12, “Evaluation of a diagnostic leaching technique for gold innative gold and gold/silver tellurides” describe a stage-1 leach in 0.1%cyanide at pH 9.5 for 24 hours, and a stage-2 leach in 2% cyanide at pH12.5 for 96 hours.

Goodall et al. in Minerals Engineering Vol 18, Issue 10, August 2005,pages 1010-1019, “Applications of PIXE and diagnostic leaching in thecharacterisation of complex gold ores” describe the use of variousdiagnostic leach sequences on source materials to provide samples foranalysis by Proton-Induced X-Ray Emission (PIXE). From these studies itwas concluded that refractory gold may be locked within a sulphidematrix.

Resource Development Incorporated offers customised diagnostic leachingwhich involves progressively more aggressive leaches and interstagecyanidation to determine deportation of gold in various mineral types.Celep et al. in DPU Fen Bilimleri Enstitusu 16 Sayi Eylul 2008“Application of Diagnostic Leaching Technique for Refractory Gold Ores”note that diagnostic leaching can attribute refractoriness in an ore tofine gold particles locked up within carbonates, oxides, sulphides andsilicates.

The key feature of the diagnostic leach technique is that thecharacterisation of a source material is achieved by specifying a leachsequence in conjunction with associated analysis protocol for successiveleach liquors. Since gold is generally present at rather low weightfractions in a source material, the removal of “easy” gold and “easy”matrix materials prior to the performance of more invasive leachingsteps designed to liberate more refractory gold is a logical approach tocharacterising gold-containing materials, an approach that minimisesanalytical ambiguity (and in particular minimises analyticaldifficulties that may arise when one component in the materialinterferes in the analysis of another component).

Diagnostic leaching provides an efficient framework for defining novelprocess steps that are effective in releasing refractory gold.Diagnostic leaching furthermore provides a framework for characterisingcomponents of source materials that interfere with gold recovery. Inprior art, such interfering components have been considered to bepresent in significant excess (as weight fraction in source material)over gold. There has been little attempt to address interferingcomponents that are present at low levels such as less than 1000 partsper million and especially less than 100 parts per million (by weight).

In some instances the presence of refractory gold in a material mayremain undiscovered until a means is found of exposing the presence ofinterferent and of removing the interferent. In these cases the goldcontent of the material may be substantially masked or undervalued.

In summary, the reasons for the routine use of diagnostic leaching thegold industry include:

-   -   Avoidance of the nugget effect;    -   Minimisation of interferences in analytical procedures;    -   Capacity to characterise material responsible for inhibiting        gold recovery; and

Capacity to streamline process development by the provision of clearsignals on the impact of individual unit operations, and combinations ofunit operations on components of samples that contain gold.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention as it existed before the priority date of each claimof this application.

There is an ongoing need to develop pre-treatments that liberaterefractory gold, both for use in gold analysis and in gold recovery.

SUMMARY

The present inventor has found that in many cases it is possible torecover refractory gold by removal of interferents comprising leadmoieties which are nitric acid insoluble. The removal of suchinterferents during processing of the gold containing material can leadto gold being recovered in yield greater than would otherwise beexpected.

There is thus provided an improved method for recovering refractory goldfrom a material comprising at least partially removing nitricacid-insoluble lead moieties from the material.

In a preferred set of embodiments the method comprises treating thematerial to transform at least a portion of the nitric acid insolublelead moieties into leachable lead moieties and leaching the material toremove at least some of the transformed lead moieties.

Examples of processes adapted to at least partially transform nitricacid-insoluble lead moieties from the gold containing material intoleachable lead moieties include contacting the gold containing materialwith an agent selected from the group consisting of reducing agents,lead solubilising agents and lead complexing agents. It may be the casethat that the effectiveness of an agent for removing nitricacid-insoluble lead moieties or for transforming nitric acid-insolublelead moieties into leachable lead moieties can be enhanced by (a)providing intensive agitation or cavitating agitation to thegold-containing source material, or (b) adopting a process to remove orpartially dislodge surface-residing scale. Said process may include theuse of heat treatments to evaporatively remove liquor from the surfaceof the source materials and/or contacting the source material withorganic solvents.

The present inventor has found that the use of ultrasonic agitation isparticularly effective for enhancing the capacity of an agent to removenitric acid-insoluble lead moieties or to transform nitricacid-insoluble lead moieties into leachable lead moieties.

The present inventor has also found that the use of a heated agent iseffective for enhancing the capacity of said agent to remove nitricacid-insoluble lead moieties or to transform nitric acid-insoluble leadmoieties into leachable lead moieties.

The above agents may be gaseous or liquid agents. The gaseous agent maybe dissolved in a liquid carrier, and may be stabilised by keeping saidcarrier under pressure. The gaseous agent may comprise ammonia or otheramine functionality.

If the above-mentioned agent is a liquid, it may be an aqueous liquid.The aqueous liquid may provide a reducing environment or an oxidisingenvironment, or an environment which is neither reducing nor oxidising.The oxidation potential of the aqueous liquor may vary from a negative(reducing) value to a less negative or even positive (oxidising) valueduring the course of the treatment. Alternatively the oxidationpotential of the liquid may vary from a positive (oxidising) value to aless positive or even negative (reducing) value during the course of thetreatment. Independently of the oxidation potential of the liquid, thepH of the liquid may be acidic, neutral or alkaline.

In one preference the aqueous liquid may comprise ammonium ions or otherions that comprise amine functionality.

One or a plurality of sequential processes may be used to at leastpartially remove nitric acid-insoluble moieties from the sourcematerial.

In one preference, fine particles are at least partially removed fromthe source material prior to treatment to at least partially removenitric acid-insoluble moieties from the source material. In onepreference, fine particles are at least partially removed by passing a10% slurry of source material through a 2-inch Mozley hydrocylone at 350kPa input pressure, rejecting the overflow stream and collecting thatfraction of source material in the underflow stream for furthertreatment.

Many processes that involve contacting gold containing material withreducing agents, lead solubilising agents, lead complexing agents areineffective in removing nitric acid-insoluble lead moieties. This isbecause nitric acid is a powerful reagent for extracting lead moietiesfrom a sample.

The invention provides a method for identifying suitable agents andconditions for at least partially removing nitric acid-insoluble leadmoieties from a sample. Suitable agents and conditions can beestablished using the “nitric acid-insoluble lead detection test” asherein described. This test involves (a) a first step of providingmultiple nitric acid leaching steps to a sample until the level of leadsolubilised in the final nitric acid leaching liquor is reliably lessthan 5 ppm, preferably less than 1 ppm, (as a weight fraction of theinitial sample weight)—this leaves an ultimate residue devoid of nitricacid-soluble lead moieties and (b) a second step of applying candidateagents and conditions to the ultimate residue—those agents andconditions that facilitate the liberation of more than 1.5 ppm lead fromthe ultimate residue are recognised as having enabled the at leastpartial removal of nitric acid-insoluble lead moieties from the sample.

An important aspect of the invention is that agents and conditionsidentified in the above test may be applied directly to source materialas a treatment to enable the recovery of refractory gold from the sourcematerial.

Examples of suitable reducing conditions include reducing conditionsprovided by an electrode in contact with a liquor, and/or reducingconditions provided by the use of reducing agents, and/or reducingconditions provided by the use of reducing micro-organisms.

Reducing agents are preferably chosen from the set of reducing agentscompatible with water. Preferably these reducing agents are chosen fromthe set comprising chromium(II), tin(II), copper(I), titanium(II) andtitanium(III) moieties, and also comprising sulfites, sulphur-containingreducing agents oxalic acid and other organic reducing agents.

In one preference, the a suitable aqueous reducing liquor will comprisetin(II) moities, more preferably in the form of stannous chloride.

In one preference, the aqueous reducing liquor acidic, and preferablythe pH is less than 1.

Examples of lead complexing or lead solubilising agents may be aqueousliquors comprising one or more selected from the group consisting ofhydrochloric acid, nitric acid, alkaline material such as sodiumhydroxide or other hydroxide moities or other water-compatible alkalis,lead acetate, ammonium chloride, chlorides, carboxylic acids and theirsalts, chelating agents, fluoro silicate, phenol sulfonate,peroxy-disulfate and any other agent that enhances the solubility oflead oxide moieties in water, When the lead complexing or solubilisingagent is selected from carboxylic acids and their salts or chlorides itis preferred that (a) the carboxylic acids are selected from the groupconsisting of citric acid, lactic acid, acetic acid, formic acid,iso-butyric acid, acetyl salicylic acid and their salts such as thealkali and alkaline earth metal salts and (b) the chlorides are selectedfrom the group consisting of ammonium chloride, sodium chloride,potassium chloride, calcium chloride and strontium chloride.

In a further set of embodiments there is provided a method ofdetermining the presence of refractory gold in a material suspected ofcontaining refractory gold comprising separating a sample from thematerial; subjecting the sample to a treatment adapted to at leastpartially remove nitric acid-insoluble lead moieties from the sample,and analysing the treated sample for gold.

In this set of embodiments we have found that when the sample, aftermultiple nitric acid leaching steps releases less than 1 ppm of lead inthe last nitric acid leach liquor, and when the treatment adapted to atleast partially remove nitric acid-insoluble lead moieties liberates atleast 1.5 ppm lead from the residue formed after the nitric acid leachsequence, this is a good indication that the said treatment will beeffective in enabling refractory gold to be analysed in the sample.

Accordingly, the method may comprise determining the presence ofrefractory gold and recovering the refractory gold by treating thematerial with the method adapted to solubilise nitric acid insolublelead moieties.

DETAILED DESCRIPTION

The present inventor has investigated a variety of gold-containingsource materials using diagnostic leach methodology. They haveparticularly investigated source materials using a diagnostic leachprotocol comprising a sequence of nitric acid leaches. Residues fromsuch multiple nitric acid leach treatments will be denoted “ultimateresidues” in accordance with the prior art nomenclature. The prior artstates that any gold remaining in ultimate residues will be locked in asilica matrix, and can only be un-locked by methods that provide matrixerosion of the silica matrix, e.g. treatment with hydrofluoric acid.

The inventors have made the surprising discovery that refractory goldmay exist in ultimate residues in association with lead moieties. Suchlead moieties are not locked in a silica matrix and are noteworthy inthat they are insoluble in a succession of nitric acid leachingsteps—henceforth they will be referred to as “nitric acid-insoluble leadmoieties”. The association between refractory gold and nitricacid-insoluble lead moieties has not previously been identified.

Notwithstanding that the at least partial removal of nitric acidinsoluble lead moieties may be important to the recovery of refractorygold there is no requirement to conduct nitric acid leach steps in themethod of the invention. Diagnostic leaching may, if desired, be used toidentify the presence of nitric acid insoluble lead and may also beuseful in conducting trials to examine agents and processes for the atleast partial removal of nitric acid insoluble lead moieties.

Nitric acid insoluble lead moieties have a significant effect on theability to recover gold even when they are present at low levelsrelative to nitric acid soluble lead moieties. For example we have foundthe nitric acid insoluble lead moieties, when transformed andsolubilised in a leach liquor are frequently present at less than 10%such as less than 5%, less than 2% and less than 1% of the cumulativeamount of nitric acid soluble lead moieties that can be recovered insuccessive nitric acid leaching steps.

Definitions

“refractory-gold materials” refers to source materials containing goldwhich can not be recovered by direct cyanide treatment.

“refractory gold” refers to gold that is present in refractory sourcematerials but which is not amenable to removal by direct cyanidation Adiscussion of refractory source materials is provided in U.S. Pat. No.5,232,490 (Bender et al., filed 1992), the contents of which areincorporated by reference.

“matrix erosion pre-treatments” refers to pre-treatments such ashydrometallurical and pyrometallurgical pretreatments which lead to thedissolution/erosion of a significant fraction of the source material(more than 15% by weight, frequently more than 20% by weight, calculatedon the weight of water-washed and dried source material).

The term “nitric acid-insoluble lead moieties” refers to lead moietieswhich are insoluble in a succession of nitric acid leaching steps. Thesemoieties may act as interferents in conventional gold recoveryprocesses.

The terms “ultimate nitric acid leach residues” and “ultimate leachresidues” refer to residues from such multiple leach treatments withnitric acid.

Throughout the description and the claims of this specification the word“comprise” and variations of the word, such as “comprising” and“comprises” is not intended to exclude other additives, components,integers or steps.

The inventors have found that treatments of ultimate residue that atleast partially remove nitric acid-insoluble lead moieties may also beapplied directly to material believed to contain refractory gold astreatments or pre-treatments for recovering refractory gold.

Without wishing to be bound by theory, the chemical nature of the nitricacid-insoluble lead moieties that have been discovered by the presentinventor to interfere with gold recovery are believed to includemoieties encompassed by the term “sulphur-deficient lead sulfate”. Onesuch moiety is the mineral lanarkite, however, it is believed that goldmay also be in association with other sulphur-deficient lead sulfatemoieties, and that said moieties may be present as a coating thatresists dissolution and that surrounds microfine dispersed gold in thesource materials.

Agents and conditions for at least partially removing nitric acidinsoluble lead from a given material may be chosen without undueexperimentation by examination of the material using the Nitricinsoluble lead detection test described below. Once suitable conditionsfor at least partially removing nitric acid insoluble lead moieties havebeen determined the given material may be processed using the chosenconditions (i.e. without the use of nitric acid leaches).

Nitric Insoluble Lead Detection Test Part 1—Diagnostic Leach ProcedureFor Generation of Standard Ultimate Residue

Particulate source material, generally less than 500 microns in size,(CHECK) is water washed and dried. 10 g of this material is placed in a600 ml beaker containing 200 mls of liquor comprising 1 part by volumewater and 1 part by volume of conc. nitric acid. The beaker is placed inthermostatted ultrasonic bath at 60° C. (bath frequency 42 kHz) with thepower set at 150 Watts (i.e. 60% of maximum 250 Watt power). Ultrasonicagitation was applied for one hour followed by one hour with the poweroff, then one hour with power on etc. for a total of eight hours. Theresidue is then filtered and the filtrate tested for lead using flameAA. The entire residue is again leached in 50% conc. nitric acidaccording to the above protocol, and the filtrate again assayed forlead. Further leaches are carried out until the lead in filtrate(expressed as a fraction of the weight of initial water-washed and driedsource material) is less than 1 ppm in 2 successive leach steps. Up tonine leaches or more may be applied, until the lead found in filtrate isless than 1 ppm on 2 successive leach steps.

Part 2—Evaluation of Candidate Treatment For At Least Partially RemovingNitric Acid Insoluble Lead Moieties

Standard ultimate residue is generated from the source material asoutlined in Part 1 above and the candidate treatment is applied to theresultant ultimate residue. The last step in the candidate treatment isan aqueous leaching step. Liquor for this aqueous leaching step isfiltered off and assayed for lead using flame AA. If lead in filtrate(expressed as a fraction of the weight of the initial water-washed anddried source material) exceeds 1.5 ppm this is taken as confirmationthat the candidate treatment at least partially removes nitricacid-insoluble lead moieties from the source material. The power of thediagnostic leach process is that sequential leaches of the same sampleof source material are used to establish all aspects of thecharacterisation—in particular there is no need to prepare two“identical” samples of source material for comparison. This overcomesthe well-known problem designated the nugget effect—it is that when goldis present at low levels, a chance occurrence of a gold-rich localregion in one sample cannot be replicated in another sample. An analogueof the nugget effect also applies to other materials that may be presentat low levels, and may associate with gold in such a way that recoveryof the gold is prevented. Such low-level interfering materials (e.g.nitric acid-insoluble lead moieties) will also be subject to the nuggeteffect, so that attempts at direct analytical determination lead toambiguous results. The use of diagnostic leaching also overcomesproblems associated with the analogue nugget effect for interferingmaterials. An important preference for performance of the inventionincludes the use of diagnostic leach methodology to characterise goldrecovery treatments.

Criterion For Establishing That A Candidate Treatment At Least PartiallyTransforms Nitric Acid-Insoluble Lead Moieties To Nitric Acid-SolubleLead Moieties In A Source Material

Standard ultimate residue is generated from the source material asoutlined above and the candidate treatment is applied to the ultimateresidue. Thereafter the resulting residue is leached in a liquor capableof dissolving lead (e.g. 50% nitric acid in water according to theprotocol provided in the above section on diagnostic leach procedure),and the filtrate is assayed for lead using flame AA or an equivalentaccredited method. If lead in filtrate after this final leach (expressedas a fraction of the weight of the initial water-washed and dried sourcematerial) exceeds 1.5 ppm, this is taken as confirmation that thetreatment at least partially converts nitric acid-insoluble leadmoieties to nitric acid-soluble lead moieties in a source material.

Definition of “Indicative Level of Nitric Acid-Insoluble Lead Moieties”

The total quantity of nitric acid-insoluble lead moieties present in asample may be difficult to determine in an absolute and unambiguousmanner, in the same way as the total level of refractory gold in asample is difficult to determine. This is because whatever the quantityof nitric acid-insoluble lead moieties recovered from treated ultimateresidue in a leach liquor, further quantities may still reside in theresidue. However the inventors have found that if a standard ultimateresidue is generated from a sample of source material, a leach (underultrasonic agitation at 60 deg C as described above in the section onDiagnostic Leach Procedure for generation of Standard Ultimate Residue)of the standard ultimate residue with a liquor made by adding 100 mlconc HCl to 100 ml water, and dissolving 8 g stannous chloride dihydratein the resultant 50% HCl frequently liberates significantly more leadinto the leach liquor than was liberated in the previous nitric acidleach (used to generate the standard ultimate residue).

The excess of lead from the acid/stannous chloride leach over thepreceding nitric acid leach (expressed as a fraction of originalwater-washed and dried source material in ppm) will, here and henceforthbe designated as the indicative level of acid-insoluble lead moieties inthe source material. The indicative level of nitric acid-insoluble leadmoieties in a sample is a quantity that is well-defined in terms ofdiagnostic leach procedures, although clearly the content ofacid-insoluble lead moieties in a sample may be greater than theindicative level.

The fire assay is a common technique used to estimate the gold contentof a source material and is the subject of Australian Standard 2917.2(1994) “Fire assay gravimetric and atomic absorption spectrometricmethod” and coresponding international standards. The presence ofrefractory gold is generally demonstrated by noting that the gold yieldfrom cyanidation is significantly less, in percentage terms, than thegold yield by fire assay. Fire assays are usually conducted byspecialist laboratories such as Amdel Laboratories in Australia.

In some instances the present inventor has found that pre-treatmentswhich at least partially remove nitric acid-insoluble lead moieties fromthe material can lead to recovery higher than indicated by fire assay ofthe material.

Thus, pretreatment can lead to the recovery of more gold (in asubsequent leach or leach sequence) than the gold yield in the fireassay of the original material.

In a preferred set of embodiments the method comprises treating thematerial to transform at least a portion of the nitric acid insolublelead moieties into leachable lead moieties and leaching the material toremove at least some of the transformed lead moieties.

In one preference, there is provided an improved method for recoveringrefractory gold from a material, wherein the source material comprisesboth nitric acid-soluble lead moieties and an indicative level of nitricacid-insoluble lead moieties, and wherein the ratio of nitricacid-soluble lead moieties to the indicative level of nitricacid-insoluble lead moieties is greater than 10:1, more preferablygreater than 100:1.

In one preference, the method involves relatively minor erosion of thematrix of the material treated to remove nitric acid insoluble leadmoieties (based on weight of water washed and dried matrix) andpreferably less than 10%, such as less than 5%, less than 2% and lessthan 1%. Less preferred matrices involve those that undergo significanterosion. For example, a mineral acid used on a carbonate matrix, wouldcause significant corrosion. Specifically, the preferred method involvesrelatively minor erosion of the matrix as compared to the same leachingsolution in the absence of reducing components (eg. HCl/stannous leachas compared with an HCl leach alone)

In one preference, the method of the invention involves treating thesource material with a reducing aqueous liquor or a reducing gas. Thereducing leach may produce at least partial removal of a base metal fromthe source material. Without being bound by theory it is believed likelythat the use of a reducing leach may facilitate the dissolution ofmoieties comprising Iron (III), and that these moieties are responsibleor partially responsible for immobilizing gold. Evidence for thedissolution of moieties comprising iron III includes decoloration ofmaterial after leaching. Leaching may be carried out in liquorscomprising 1% HCl and one or more reducing agents such as tin (II)chloride, chromium (II) chloride and oxalic acid. Based on the observeddegree of decoloration the effectiveness of reducing agents decreasesaccording to the ranking tin (II) chloride, ≧chromium (II)chloride>oxalic acid

The invention will now be described with reference to the followingexamples. It is to be understood that the examples are provided by wayof illustration of the invention and that they are in no way limiting tothe scope of the invention.

EXAMPLES Example 1

This example shows how ultimate residue is generated from a sourcematerial.

1 part by weight of fine playa source material (sub 100 microns) wastaken from near Silver City in Mew Mexico USA and was slurried in water(10 parts by weight), then the slurry was passed through a 2-inch Mozleyhydrocyclone at 350 kPa pressure. The overflow stream was rejected, andthe underflow stream was filtered under air pressure to form a moistresidue which was placed in an oven at 80 deg C overnight to dry, givinga de-slimed dried, water-washed material for further treatment,

This material was subject to a succession of nitric acid leaching stepswith ultrasonic agitation, according to the following protocol:

L1: 10 g of de-slimed washed and dried playa material was placed in a600 ml beaker containing a liquor made by adding 100 ml of concentratednitric acid to 100 ml water. The beaker was placed in a Sonicleanthermostatted ultrasonic bath (250 W max power setting, bath frequency42 kHz). The ultrasonic bath temperature was set at 60 deg C, and thepower setting was 60%, corresponding to 150 W power. Ultrasonic powerwas applied for one hour, followed by one hour power-off, then one hourpower on and so forth, for a total of 8 hours bath time. The slurry inthe beaker was filtered using Whatman no 1 filter paper, and thefiltrate was taken for lead analysis by AA. The lead content of filtrate(expressed as a fraction of the 10 g of starting material) was 37.1 ppm.

L2: The moist residue on the filter paper from L1 was quantitativelytransferred to a second 600 ml beaker and to it was added of 100 ml ofwater and 100 ml of concentrated nitric acid. Thereafter the protocolwas as for L1. The filtrate was taken for lead analysis (by AA) andfound to contain 3.12 ppm lead (expressed as a fraction of the 10 g oforiginal starting material).

L3: Moist residue from L2 was quantitatively transferred to a third 600ml beaker and to it was added 100 ml of water and 100 ml of concentratednitric acid. Thereafter the protocol was as for L1. The filtrate wastaken for lead analysis (by AA) and found to contain 1.01 ppm lead(expressed as a fraction of the 10 g of original starting material).

L4: Moist residue from L3 was quantitatively transferred to a fourth 600ml beaker and to it was added 100 ml of water and 100 ml of concentratednitric acid. Thereafter the protocol was as for L1. The filtrate wastaken for lead analysis (by AA) and found to contain 0.65 ppm lead(expressed as a fraction of the 10 g of original starting material).

L5: Moist residue from L4 was quantitatively transferred to a fifth 600ml beaker and to it was added 100 ml of water and 100 ml of concentratednitric acid. Thereafter the protocol was as for L1. The filtrate wastaken for lead analysis (by AA) and found to contain 0.0 ppm lead(expressed as a fraction of the 10 g of original starting material).

The resultant residue was designated ultimate residue derived from thestarting material, since nitric acid leaching with ultrasonic agitationwas found on 2 successive occasions to liberate less than 1 ppm leadinto the leach liquor. This ultimate residue was taken to besubstantially free of nitric acid-soluble lead moieties.

In repeat experiments with samples of the same de-slimed washed anddried playa material, it was found that 5 successive nitric acidleaching steps with ultrasonic agitation was sufficient to generateultimate residue—in each experiment the last 2 sequential leach stepsliberated less than 1 ppm lead into the leach liquor.

Example 2

This example illustrates a procedure, according to the currentinvention, for identifying agents and conditions that enable the atleast partial removal of nitric acid-insoluble moieties from a sample.The sample is the playa material described in example 1. The basis ofthe procedure is to first rigorously remove nitric acid-soluble leadmoieties from the sample, and then to search for the appearance of leadin subsequent trial leaching steps. In this example, the trial leachingsteps involve the use of stannous chloride in aqueous acid conditions.

Ultimate residue was generated from 10 g de-slimed washed and driedplaya material as described in example 1. The ultimate residue wastransferred to a 600 ml beaker and to it was added an aliquot of liquorconsisting of (a) 100 ml water; (b) 100 ml concentrated hydrochloricacid; and (c) 8 g stannous chloride dihydrate (dissolved). Stannouschloride is a known reducing agent. The beaker was placed in a Sonicleanthermostatted ultrasonic bath (250 W max power setting, bath frequency42 kHz). The ultrasonic bath temperature was set at 60 deg C, and thepower setting was 60%, corresponding to 150 W power. Ultrasonic powerwas applied for one hour, followed by one hour power-off, then one hourpower on and so forth, for a total of 8 hours bath time. The slurry inthe beaker was filtered using Whatman no 1 filter paper, and thefiltrate was taken for lead analysis by ICP (samples were sent to VeoliaLaboratories in Adelaide, Australia). The lead content of filtrate(expressed as a fraction of the 10 g of original washed and dried playasource material) was 21 ppm. This quantity of lead was significantlygreater than the lead (less than 1 ppm) found in the ultimate andpenultimate nitric acid/ultrasonic leach steps used to generate ultimateresidue. In other words, the above-described reducing acid leach withhydrochloric acid and stannous chloride led to the at least partialremoval of nitric acid-insoluble lead moieties from the ultimateresidue. In this case, the indicative level of nitric acid-insolublelead (as previously defined) in the de-slimed washed and dried playasource material was 21 ppm.

The residue from the filter paper obtained after the above-describedacid reducing leach was transferred to a 600 ml beaker, and to it wasadded 200 mls of 4% aqueous sodium hydroxide solution. The beaker wasplaced in the ultrasonic bath and subjected to the leaching protocol inthe bath for 8 hours as described previously. After theleaching/ultrasonic treatment was finished, the slurry was filteredthrough a Whatman 1 filter paper, and the filtrate analysed for lead byflame AA. The lead content of filtrate (expressed as a fraction of the10 g of original washed and dried playa source material) was 6.5 ppm.From this result it is clear that the above-described sequence of (a) aleaching step under acid reducing conditions, and (b) a leaching stepunder alkaline conditions is particularly effective in removing nitricacid-insoluble lead moieties from a sample.

Caustic Liquor Leaching Does Not Remove Nitric Acid-Insoluble LeadMoieties From Ultimate Residue Generated From This Source Material

Ultimate residue from a 10 g sample of de-slimed washed and dried playamaterial (prepared as described in example 1) was directly leached in 4%aqueous sodium hydroxide solution as described above. After theleaching/ultrasonic treatment was finished, the slurry was filteredthrough a Whatman 1 filter paper, and the filtrate analysed for lead byflame AA. The lead content of filtrate (expressed as a fraction of the10 g of original washed and dried playa source material) was less than 1ppm. This experiment shows that leaching with ultrasonic agitation usingaqueous sodium hydroxide liquor does NOT lead to the at least partialremoval of nitric acid-insoluble lead moieties. The above-describedappearance of lead in the alkaline leach liquor has occurred after aprior leaching step in acid reducing conditions (see above). This can berationalised as follows: under acid/reducing leach conditions, some ofthe nitric acid-insoluble lead moieties are transformed into (leachable)lead moieties that can be leached into caustic liquor.

According to the teaching of the present invention, the above-describedsequence of (a) a leaching step under acid reducing conditions, and (b)a leaching step under alkaline conditions is adapted to provide apre-treatment of source material that will enable the subsequentrecovery of refractory gold (see example 3).

Example 3

This example shows how the use of agents and conditions that at leastpartially remove nitric acid-insoluble lead moieties (see example 2) canenable the recovery of refractory gold. The agents and conditionscomprise a) a leaching step under acid reducing conditions, and (b) aleaching step under alkaline conditions. Both steps include the use ofultrasonic agitation.

10 g of de-slimed washed and dried playa material (described inexample 1) was sent to a specialist gold assay laboratory (Amdel) forthe fire assay determination of gold. The gold assay result was 0.3 ppm.

Another sample consisting of 10 g of de-slimed washed and dried playamaterial was given the following pre-treatment (see example 2): thesample was transferred to a 600 ml beaker and to it was added liquorcomprising (a) 100 ml water; (b) 100 ml concentrated hydrochloric acid;and (c) 8 g stannous chloride dihydrate (dissolved). The beaker wasplaced in a Soniclean thermostatted ultrasonic bath (250 W max powersetting, bath frequency 42 kHz). The ultrasonic bath temperature was setat 60 deg C, and the power setting was 60%, corresponding to 150 Wpower. Ultrasonic power was applied for one hour, followed by one hourpower-off, then one hour power on and so forth, for a total of 8 hoursbath time. The resultant slurry in the beaker was filtered using Whatmanno 1 filter paper, and the residue from the filter paper was transferredto a 600 ml beaker, and to it was added 200 mls of 4% aqueous sodiumhydroxide liquor. The beaker was placed in the Soniclean ultrasonic bathand treated according to the ultrasonic bath protocol described above.As noted in example 2, the above combination of steps accomplished theat least partial removal of nitric acid-insoluble moieties from thesource material.

Gold Recovery

The resultant slurry was filtered using a Whatman 1 filter paper, andthe residue was assayed for gold content using the following aqua regiaprotocol: 200 ml concentrated hydrochloric acid and 50 ml concentratednitric acid were added to a 600 ml beaker, and the residue was promptlyadded. The beaker was added to the ultrasonic bath (60 deg C) andagitated for 2 minutes at 250 W (full power). The beaker was placed on ahot plate and heated to incipient boiling for 5 minutes, then added tothe ultrasonic bath again with agitation at 250 W for another 2 minutes.The beaker was placed on a hot plate and heated to incipient boiling yetagain for 5 minutes, then added yet again (a third time) to theultrasonic bath with agitation at 250 W power for 2 minutes. The beakerwas then returned to the hot plate and boiled until (a) evolution ofnitrous fumes ceased, and (b) the liquor volume in the beaker was downto 80 ml. The slurry in the beaker at this point was hot-filteredthrough a Whatman 1 filter paper, and the filtrate was retained for AAanalysis for gold. A gold content of 7.0 ppm was found (expressed as afraction of the 10 g of original washed and dried playa sourcematerial). Compared with the gold assay value discovered by standardfire assay (0.3 ppm), it can be seen that the method of the inventionprovides a pre-treatment that enables recovery of refractory gold thatis significantly in excess of the fire assay content.

Example 4

This example shows how agents and conditions quite distinct from thosedescribed in example 2 can be used to at least partially remove nitricacid-insoluble lead moieties from a particular sample. As in example 2,the appropriate agents and conditions are explored by first rigorouslyremoving nitric acid-soluble lead moieties from the sample, and thensearching for the appearance of lead in subsequent trial leaching steps.The example shows a 2-step trial process wherein (a) a first leach withaqueous ammonia transforms nitric acid-insoluble lead moieties toleachable form (however lead moieties are not liberated into the ammonialiquor); and (b) a second leach with aqua regia liberates the leachablelead moieties formed in the first leach. Both leaches were performedwith ultrasonic agitation.

Ultimate residue was generated from 10 g de-slimed washed and driedplaya material as described in example 1. The ultimate residue wastransferred to a 600 ml beaker and to it was added liquor consisting of200 mls 7.5% aqueous ammonia. The beaker was placed in a Sonicleanthermostatted ultrasonic bath (250 W max power setting, bath frequency42 kHz). The ultrasonic bath temperature was set at 60 deg C, and thepower setting was 60%, corresponding to 150 W power. Ultrasonic powerwas applied for 10 minutes, followed by 2 hours power-off, then tenminutes power on and so forth, for a total of 8.5 hours bath time. Theslurry in the beaker was filtered using Whatman no 1 filter paper, andthe filtrate was taken for lead analysis by AA. The lead content offiltrate (expressed as a fraction of the 10 g of original washed anddried playa source material) was 0.92 ppm. This quantity of lead was NOTsignificantly greater than the lead (less than 1 ppm) found in theultimate and penultimate nitric acid/ultrasonic leach steps used togenerate ultimate residue.

The residue from the above ammonia leach was treated using the aquaregia protocol described in experiment 3 above, except that the finalliquor was assayed for lead rather than gold (aqua regia has aconsiderable capacity for solubilising lead, albeit less than nitricacid). A lead content of 10.23 ppm was found (expressed as a fraction ofthe 10 g of original washed and dried playa source material)—this issignificantly greater than the lead found (less than 1 ppm) in thepenultimate and ultimate nitric acid/ultrasonic leach steps used togenerate ultimate residue.

This result shows that the ammonia leaching step described aboveconverts nitric acid-insoluble lead moieties in the residue to leachablelead moieties (that were at least partially removed by the aqua regialeach).

According to the teaching of the present invention, the above-describedammonia leach, in conjunction with the use of a leach that removesleachable lead moieties (formed in the ammonia leach), is adapted toprovide a pre-treatment of source material that will enable thesubsequent recovery of refractory gold (see example 5).

Example 5

This example shows how washed and dried playa source material can betreated with an ammonia leach followed by a further aqueous leachingstep that removes (leachable) lead formed in the ammonia leach. Thistreatment at least partially removes nitric acid-insoluble lead moieties(see example 4) and is shown to enable the recovery of refractory gold.

10 g of de-slimed washed and dried fine playa source material was sentto a specialist gold assay laboratory (Amdel) for the fire assaydetermination of gold. The gold assay result was 0.3 ppm (average ofduplicate samples).

Another sample consisting of 10 g of de-slimed washed and dried playamaterial was given the following pre-treatment (see example 4): Thesample was transferred to a 600 ml beaker and to it was added liquorconsisting of 200 mls 7.5% aqueous ammonia. The beaker was placed in aSoniclean thermostatted ultrasonic bath (250 W max power setting, bathfrequency 42 kHz). The ultrasonic bath temperature was set at 60 deg C,and the power setting was 60%, corresponding to 150 W power. Ultrasonicpower was applied for 10 minutes, followed by 2 hours power-off, thenten minutes power on and so forth, for a total of 8.5 hours bath time.The slurry in the beaker was filtered using Whatman no 1 filter paper,and the residue was analysed for gold using the aqua regia protocoldescribed in experiment 3. A gold content of 3.5 ppm was found(expressed as a fraction of the 10 g of original washed and dried playasource material). Compared with the gold assay value discovered bystandard fire assay (0.3 ppm), it can be seen that the method of theinvention provides a pre-treatment that enables recovery of refractorygold that is significantly in excess of the fire assay content.

Example 6

This example illustrates the use of the present invention on a differentstarting material. The starting material was designated CH-4 by CANMETMining and Mineral Sciences Laboratories, 555 Booth St, Ottawa, Ontario,Canada. The sample was characterised as part of the Canadian CertifiedReference Materials Project. The source material for CH-4 was donated byCorporation Miniere Inmet, Division Troilus, Chibougama, Quebec in 2000.After crushing, milling, sieving and blending, the yield was 37%. Thematerial is a fine powder with a mesh size of less than 45 microns (325mesh). The host rock of the raw material is meta-anorthosite. Themineralogy includes pyrrhotite, pyrite and chalcopyrite, and smallamounts of spalerite, galena and molybdenite. The mean gold content ofthe CH-4 CANMET standard is given as 0.88 ppm, with a within-labstandard deviation of 0.04 ppm, and a between-labs standard deviation of0.04 ppm.

The procedure was to first rigorously remove nitric acid-soluble leadmoieties from CANMET sample CH-4, and then to search for the appearanceof lead in subsequent trial leaching steps. In this example, the trialleaching steps involve the use of stannous chloride in aqueous acidconditions.

Ultimate residue was generated from 10 g CANMET CH-4 material asdescribed in example 1. No detectable lead was found in the 4^(th) and5th nitric acid leach liquors. The ultimate residue was transferred to a600 ml beaker and to it was added an aliquot of liquor consisting of (a)100 ml water; (b) 100 ml concentrated hydrochloric acid; and (c) 8 gstannous chloride dihydrate (dissolved). Stannous chloride is a knownreducing agent. The beaker was placed in a Soniclean thermostattedultrasonic bath (250 W max power setting, bath frequency 42 kHz). Theultrasonic bath temperature was set at 60 deg C, and the power settingwas 60%, corresponding to 150 W power. Ultrasonic power was applied forone hour, followed by one hour power-off, then one hour power on and soforth, for a total of 8 hours bath time. The slurry in the beaker wasfiltered using Whatman no 1 filter paper, and the filtrate was taken forlead analysis by ICP (samples were sent to Veolia Laboratories inAdelaide, Australia). The lead content of filtrate (expressed as afraction of the 10 g of CH-4 starting material) was 14 ppm. Thisquantity of lead was significantly greater than the lead (less than 1ppm) found in the ultimate and penultimate nitric acid/ultrasonic leachsteps used to generate ultimate residue. In other words, theabove-described reducing acid leach with hydrochloric acid and stannouschloride led to the at least partial removal of nitric acid-insolublelead moieties from the ultimate residue. In this case, the indicativelevel of nitric acid-insoluble lead (as previously defined) in thede-slimed washed and dried playa source material was 14 ppm.

The residue from the filter paper obtained after the above-describedacid reducing leach was transferred to a 600 ml beaker, and to it wasadded 200 mls of 4% aqueous sodium hydroxide solution. The beaker wasplaced in the ultrasonic bath and subjected to the leaching protocol inthe bath for 8 hours as described previously. After theleaching/ultrasonic treatment was finished, the slurry was filteredthrough a Whatman 1 filter paper, and the filtrate analysed for lead byflame AA. The lead content of filtrate (expressed as a fraction of the10 g of original washed and dried playa source material) was 2.8 ppm.From this result it is clear that the above-described sequence of (a) aleaching step under acid reducing conditions, and (b) a leaching stepunder alkaline conditions is particularly effective in removing nitricacid-insoluble lead moieties from a sample.

Example 7

This example is based on an investigation of CANMET sample CH-4. Theexample shows how the use of agents that at least partially removenitric acid-insoluble lead moieties (see example 6) can enable therecovery of refractory gold. An investigation is made of the use of acidreducing conditions to liberate refractory gold from starting material.

CH-4 material contained 0.88 ppm gold on the basis of conventional goldassays (see example 6).

A sample consisting of 10 g of CANMET CH-4 material was given thefollowing pre-treatment: the sample was transferred to a 600 ml beakerand to it was added liquor comprising (a) 100 ml water; (b) 100 mlconcentrated hydrochloric acid; and (c) 8 g stannous chloride dihydrate(dissolved). The beaker was placed in a Soniclean thermostattedultrasonic bath (250 W max power setting, bath frequency 42 kHz). Theultrasonic bath temperature was set at 60 deg C, and the power settingwas 60%, corresponding to 150 W power. Ultrasonic power was applied forone hour, followed by one hour power-off, then one hour power on and soforth, for a total of 8 hours bath time. The resultant slurry in thebeaker was filtered using Whatman no 1 filter paper, and the residuefrom the filter paper was transferred to a 600 ml beaker, and to it wasadded 200 mls of 4% aqueous sodium hydroxide liquor. The beaker wasplaced in the Soniclean ultrasonic bath and treated according to theultrasonic bath protocol described above. As noted in example 2, theabove combination of steps accomplished the at least partial removal ofnitric acid-insoluble moieties from the source material

Gold Recovery

The resultant slurry was filtered using a Whatman 1 filter paper, andthe residue was assayed for gold content using the following aqua regiaprotocol: 200 ml concentrated hydrochloric acid and 50 ml concentratednitric acid were added to a 600 ml beaker, and the residue was promptlyadded. The beaker was added to the ultrasonic bath (60 deg C) andagitated for 2 minutes at 250 W (full power). The beaker was placed on ahot plate and heated to incipient boiling for 5 minutes, then added tothe ultrasonic bath again with agitation at 250 W for another 2 minutes.The beaker was placed on a hot plate and heated to incipient boiling yetagain for 5 minutes, then added yet again (a third time) to theultrasonic bath with agitation at 250 W power for 2 minutes. The beakerwas then returned to the hot plate and boiled until (a) evolution ofnitrous fumes ceased, and (b) the liquor volume in the beaker was downto 80 ml. The slurry in the beaker at this point was hot-filteredthrough a Whatman 1 filter paper, and the filtrate was retained for AAanalysis for gold. A gold content of 3.0 ppm was found (expressed as afraction of the 10 g of starting material). Compared with the gold assayvalue discovered by standard fire assay (0.88 ppm), it can be seen thatthe method of the invention (using acid reducing leach conditions)provides a pre-treatment that enables recovery of refractory gold thatis significantly in excess of the fire assay content.

1. An improved method for recovering refractory gold from a materialcomprising treating the material to at least partially remove nitricacid-insoluble lead moieties.
 2. A method according to claim 1comprising treating the material to transform at least a portion of thenitric acid insoluble lead moieties into leachable lead moieties andleaching the material to remove the leachable lead moieties.
 3. A methodaccording to claim 1 wherein the process of at least partially removingnitric acid-insoluble lead moieties from the gold containing materialcomprises contacting the gold containing material with an agent selectedfrom the group consisting of reducing agents, lead complexing agents andlead solubilising agents.
 4. A method according to claim 1 comprisingcontacting the material with a reducing agent.
 5. A method according toclaim 4 wherein the reducing agent is selected from the group consistingof chromium(II), tin(II), copper(I), titanium(II) and titanium(III)moieties, and also comprises sulfites, sulphur-containing reducingagents, oxalic acid, and other organic reducing agents.
 6. A methodaccording to claim 1 comprising contacting the gold containing materialwith a lead complexing agent.
 7. A method according to claim 6 whereinthe lead solubilising and complexing agents are selected from the groupconsisting of carboxylic acids and their salts, chlorates, perchlorates,alkalis, chlorides, fluorosilicate, phenol sulfonate andperoxy-disulfonate.
 8. A method according to claim 7 wherein (a) thecarboxylic acids are selected from the group consisting of citric acid,lactic acid, acetic acid, formic acid, iso-butyric acid, acetylsalicylic acid and their salts and (b) the chlorides are selected fromthe group consisting of ammonium chloride, sodium chloride, potassiumchloride, calcium chloride and strontium chloride.
 9. A method accordingto claim 4 wherein the method of at least partially removing nitric acidinsoluble lead moieties further comprises at least one step selectedfrom the group consisting of (a) providing intensive agitation orcavitating agitation to the gold-containing source material preferablyultrasonic agitation, (b) adopting a process to at least partiallyremove surface scale and (c) conducting the at least partial removal ofnitric acid insoluble moieties at a temperature of at least 40° C.
 10. Amethod according to claim 9 wherein the method of at least partiallyremoving nitric acid insoluble lead moieties further comprisesultrasonic agitation.
 11. A method of recovering refractory gold from amaterial suspected of containing refractory gold comprising separating asample from the material; subjecting the sample to diagnostic leaching;determining the lead content of the filtrate from the last 2 nitric acidleaches in preparation of the ultimate leach residue and ensuring thatin both cases the lead content is less than 5 ppm; subjecting theultimate leach residue to a treatment adapted to solubilise nitric acidinsoluble lead moieties; analysing the filtrate from the treatedultimate leach residue for lead; wherein lead in the filtrate of thetreated ultimate residue exceeds 1.5 ppm and the method furthercomprises treating the material with the method adapted to solubilisenitric acid insoluble lead moieties to produce a residue depleted innitric acid insoluble lead moieties and recovering gold from the residuedepleted in nitric acid insoluble lead moieties.
 12. A method accordingto claim 11 wherein the method of at least partially removing nitricacid insoluble lead is a hydrometallurgical treatment which when appliedto the ultimate residue produces a lead in filtrate which exceeds 1.5ppm.
 13. A method according to claim 1 wherein at least partiallyremoving nitric acid insoluble provides gold recovery higher thanindicated by fire assay of the material.
 14. A method according to claim11 wherein the material comprises a non-silica interferent which remainsin the ultimate residue and comprises the nitric acid insoluble leadmoieties.
 15. A method according to claim 11 wherein the nitric acidinsoluble lead moieties, when transformed and solubilised in a leachliquor are present at less than 10% of the cumulative amount of nitricacid soluble lead moieties that can be recovered in successive nitricacid leaching of the material.
 16. A method according to claim 1 whereinthe nitric acid insoluble lead moieties comprise sulfur-deficient leadsulfate moieties.
 17. A method according to claim 1 wherein material isselected from the group consisting of playa material and copper ores.18-21. (canceled)
 22. A method according to claim 1, wherein thematerial is leached with an aqueous liquor comprising a reducing agentwhereby the aqueous liquor has a negative oxidation potential.
 23. Amethod according to claim 22 wherein the liquor has an acid pH.
 24. Amethod according to claim 23 wherein the liquor has a pH less than 1.